Various implantable medical devices have been developed for treating a number of ailments associated with body lumens. In particular, occlusive devices have been proven useful in filling vascular aneurysms, which are formed due to a weakening in the wall of an artery. Vascular aneurysms are often the site of internal bleeding and stroke. A variety of different embolic agents are known to be, at least arguably, suitable for treatment of vascular aneurysms by filling them to prevent further vessel wall weakening or rupture. Use of these agents are commonly known as “artificial vaso-occlusion.”
Over the past few years, advancements in the artificial occlusion of vessels and aneurysms have included the delivery and implantation of metal coils as vaso-occlusive devices. Implantable metal coils that are useful as artificial occlusion devices in vasculature lumens or aneurysms are herein referred to as “vaso-occlusive coils.” Vaso-occlusive coils are typically constructed of a wire made of a metal or metal alloy wound into a helix. Such vaso-occlusive coils are typically manufactured to assume a certain shape upon discharge of the device from the distal end of the catheter into a treatment site. A variety of such vaso-occlusive coils are known. For instance, U.S. Pat. No. 4,994,069, issued to Ritchart et al., discloses a flexible, preferably coiled wire for use in small vessel vaso-occlusion. Unlike vaso-occlusive coils used prior to that time, Ritchart et al. discloses using a coil that is relatively soft and is delivered to the site using a pusher within a catheter lumen. Upon discharge from the delivery catheter, the coil may undertake a number of random or pre-determined configurations useful to fill the site.
Known vaso-occlusive coils may be used for filling relatively small vessel sites, e.g., 0.5-6.0 mm in diameter. The coils themselves are described as being between 0.254 and 0.762 mm in diameter. The length of the wire making up the vaso-occlusive coil is typically 15 to 20 times the diameter of the vessel to be occluded. The wire used to make up the coils may be, for instance, 0.051 to 0.152 mm in diameter. Tungsten, platinum, and gold threads or wires are typically preferred. These coils have a variety of benefits, including the fact that they are relatively permanent, they may be easily imaged radiographically, they may be located at a well defined vessel site, and they can be retrieved, if necessary.
In addition to the various types of known space filling mechanisms and geometries of vaso-occlusive coils, other particularized features of coil designs, such as mechanisms for their delivery through catheters and implanting them in a desired occlusion site, are well know in the art. Examples of known vaso-occlusive coils categorized by their delivery mechanisms include pushable coils, mechanically detachable coils, and electrolytically detachable coils.
One example of a “pushable coil” is disclosed in Ritchart et al., discussed above. Pushable coils are commonly provided in a cartridge and are pushed or “plunged” from the cartridge into a lumen of a delivery catheter. A pusher (e.g., a wire or a pressurized fluid) advances the pushable coil through and out of the delivery catheter lumen, into the desired occlusion site.
Mechanically detachable vaso-occlusive coils are typically integrated with a pusher rod and are mechanically detached from the distal end of that pusher after exiting a delivery catheter. Examples of such mechanically detachable vaso-occlusive coils are found in U.S. Pat. No. 5,261,916 to Engelson and U.S. Pat. No. 5,250,071 to Palermo.
Examples of electrolytically detachable vaso-occlusive coils may be found in U.S. Pat. Nos. 5,122,136 and 5,354,295 issued to Guglielmi et al. In these devices, the vaso-occlusive portion of the assembly is attached to a pusher via a small, electrolytically severable joint. The electrolytically severable joint is eroded by the placement of an appropriate voltage on the core wire.
As noted above, aneurysms present a particularly acute medical risk due to the dangers of potential rupture of the thin vascular wall inherent in such aneurysms. Occlusion of aneurysms by use of vaso-occlusive coils without occluding the adjacent artery is a special challenge and is a desirable method of reducing such risk of rupture. Vaso-occlusive devices may be placed in an aneurysm in a manner described in U.S. Pat. No. 4,739,768 issued to Engelson. In particular, a microcatheter is initially steered into or adjacent to the entrance of an aneurysm, typically aided by the use of a steerable guidewire. The wire is then withdrawn from the microcatheter lumen and replaced by one or more vaso-occlusive coils, which are then advanced through and out of the microcatheter, and into the aneurysm.
However, after, or perhaps during delivery of a coil into the aneurysm, there is a risk that a portion of the coil might migrate out of the aneurysm entrance zone and into the feeding vessel. The presence of the coil in that feeding vessel may cause a highly undesirable occlusion there. Also, there is a risk that the blood flow in the vessel and aneurysm may induce movement of the coil farther out of the aneurysm, resulting in a more developed embolus in the feeding vessel.
One type of aneurysm, commonly known as a “wide neck” aneurysm, is known to present particular difficulty in the placement and retention of vaso-occlusive coils, because vaso-occlusive coils lacking substantial secondary shape strength may be difficult to maintain in position within an aneurysm no matter how skillfully they are placed. Wide neck aneurysms are herein referred to as aneurysms of vessel walls having a neck or “entrance zone” from the adjacent vessel, wherein the entrance zone has a diameter that either: (1) is at least 80% of the largest diameter of the aneurysm; or (2) is clinically observed to be too wide effectively to retain commercially available vaso-occlusive coils that are deployed using the techniques discussed above.
Certain techniques have been developed in order to deal with the disadvantages associated with embolic material migration into the parent vessel. One such technique, commonly referred to as flow arrest, involves temporarily occluding the parent vessel proximal of the aneurysm, so that no blood flow occurs through the parent vessel until a thrombotic mass has formed in the sac of the aneurysm. While this technique helps reduce the tendency of the embolic material to migrate out of the aneurysm sac, a thrombotic mass can still dissolve through normal lysis of blood. Also, occluding the parent vessel may not prevent all embolic material migration into the parent vessel. Further, in certain cases, it is highly undesirable to occlude the parent vessel even temporarily. Thus, a flow arrest technique is, at times, not effective or even not available as a treatment option.
Another approach to occlude a wide neck aneurysm is described in U.S. Pat. No. 6,168,622 (“the '622 patent”), which describes a vaso-occlusive device with a secondary shape having a bulbous body portion and an anchor. The bulbous body portion is deployed within the aneurysm while the anchor is set just outside of the aneurysm, covering the aneurysm's neck or entrance zone. As described in the '622 patent, the device may be integrally formed from a tube—clamped at both ends—of braided Nickel-Titanium (NiTi) wires. The bulbous body functions to occlude the aneurysm, while the anchor covers the entrance zone. In some cases, it may still be desirable to deploy vaso-occlusive coils with such a device, but the bulbous body of the vaso-occlusive device may not provide much space within the aneurysm to allow for insertion and deployment of coils.